1. Field of the Invention
The present invention relates to a method for producing steel material having a Zn series electroplated layer thereon. More particularly, the present invention relates to a method for producing an electroplated steel sheet provided with the above mentioned layer for an anticorrosive protection of an automobile.
2. Description of the Related Arts
The de-icing rock salt sprayed on roads in arctic districts in the winter causes corrosion of an automobile body, and the use of various plated steel sheets for an automobile body has been considered as a corrosion control measure. The corrosion of the outer surface of an automobile body is promoted by the gravel, sprayed rock salt and the like which are thrown up from the road surface and impinge on the automobiles traveling on a road (this impingement is referred to as chipping) at a speed equal to the running speed of the automobiles, i.e., from 50 to 150 km/hour. The resultant impingement force is very high, and thus the paint coating on the automobile body may peel off or flaws reaching the steel substrate may be formed. Water from melted snow or the salt in the de-icing material will seep into the peeled parts or flaws and exert a detrimental corrosive action on the steel substrate. Accordingly, for example, Zn plated steel sheets have been employed as a corrosion control counter measure, since the Zn plating has a strong sacrificial anode effect and thus can satisfactorily protect the steel sheet from corrosion, even though the flaws reach the steel sheet. However, when a coating of paint is applied on the Zn plated layer, as in the case of an automobile outer panel, the paint bulging referred to as blistering sometimes forms in the paint coating.
The corrosive environments to which the steel sheets of an automobile are exposed make it necessary to provide uncoated steel sheets with a corrosion resistance, and in addition, a corrosion resistance after coating, including blister resistance, water adherence resistance, red rust resistance, and flow rust resistance.
These corrosion resistances, which are improved by the Zn plated steel sheets, are further improved by a Zn-series alloy layer, such as a plated layer of Zn-Ni, Zn-Fe, Zn-Co, Zn-Fe-Cr, Zn-Ni-Co, Zn-Cr, Zn-Mn, Zn-Ti, Zn-Sn, Zn-Cu, Zn-Cd, Zn-Pb, and the like, a multilayer plated steel having a plurality of layers of the above alloys (i.e., superimposed plated layers of different compositions), a steel sheet with a graded plating layer (i.e., composition of a plated layer varies along the thickness of the layer), and a steel sheet with a composite electroplated layer(s) (i.e., particles of oxide, such as SiO.sub.2, TiO.sub.2, Al.sub.2 O.sub.3, and the like, as well as metallic particles of Zn, Al, Cr, and the like are incorporated in the Zn plated layer. Regarding the composite electroplated layer, Japanese examined Patent Publication No. 60-38480 discloses a Zn-oxide sol composite, Japanese Unexamined Patent Publication No. 60-141,898 discloses a Zn series alloy-oxide composite, and Japanese Unexamined Patent Publication No. 60-96786 discloses Zn-corrosion inhibiting pigments, such as BaCrO.sub.4. The composite plating with nonmetallic particles is disclosed in Japanese Unexamined Patent Publication No. 61-64899. However, the particles precipitated and dispersed in the Zn or Zn alloy matrix are difficult to dissolve in the acidic solution and have a size which is not easily soluble.
The above described Zn-series electroplatings of Zn alloy and Zn composite material (hereinafter collectively referred to as the Zn-series electroplating) have problems, as explained in detail below, in that, the internal stress of the Zn series-alloy plated sheets is higher than that of the ordinary electroplated steel sheet of Zn alone. The adherence of the former plated layer with the steel substrate is disadvantageously inferior to that of the latter plated layer. Generally, the outer surface of an automobile body has a three-layer paint coating approximately 100 .mu.m in total thickness and comprised of the cationic ED paint, the intercoat, and the top coat. The shrinkage stress generated during the baking of these coats has an affect on the plated layers, in that the post-coat substrate adherence is inferior to the unpainted adherence. Since the ambient temperature falls to around -50.degree. C., shrinkage of the paint coating occurs and multiplies the stress acting on the plated layer(s). Note, in the three layer-coating, there is a drastic lowering of the substrate adherence upon a fall in the temperature. When chipping occurs in a plated layer(s) exhibiting a lowered plating adherence, the plated layer(s) of the Zn series-alloy plated steel sheets is disadvantageously peeled off. The plating adherence can be improved by interposing a covering layer consisting of one or more of Cr, Mn, Fe, Co, Ni, Cu, In, Zn, Cd, Sn, and Pb between the steel substrate and the Zn series-alloy plated layer, as disclosed in Japanese Unexamined Patent Publication No. 59-200789. In this publication, however, the plating adherence is verified under an extrusion formation (5 mm Erichsen bulging) of an unpainted article at an ordinary temperature, i.e., a mild condition. Tetsu to Hagane (Bulletin of Iron and Steel Institute of Japan) 71 (1985), page 1273, discloses that a layer of one or more of Fe, Zn, Ni, Cu, and Sn interposed between the steel substrate and the Zn series-alloy plated layer provides a satisfactory plating adherence under the conditions of two coating layers, under the icing point, and in a Dupont impact test. However, the present inventors confirmed that the plating adherence of a Zn-series electroplating provided by the above method is unsatisfactory under the severe conditions found in arctic regions (-50.degree. C.), three-layer coating, and a high speed chipping. Furthermore, in the above described method for plating two different layers, at least two kinds of plating baths are necessary, which is not advantageous in the light of the production cost and control.